When a shaft is used to transfer power from a motor to a driven device, the shaft undergoes torsion. As the load exerted by the driven device changes, the torque on the connecting shaft also varies. In many applications it is desirable to measure the torque in order to determine the magnitude of the load. For example, if the driven device is the socket of a pneumatic wrench, it is desirable to measure the torque on the shaft driving the socket to derive the amount of force being exerted on the nut being tightened. This enables one to determine when the nut is properly tightened.
Manual torque wrenches often provided a scale divided into units of force and a pointer indicating the current amount of torque being exerted by the wrench. Another type of manual wrench includes a dial for setting a given amount of torque which when exerted by the wrench a mechanism releases the socket from being driven any further by the handle of the wrench. Neither of these mechanisms incorporated in manual wrenches lends itself to power wrenches for use by robot controlled fastening systems. Not only is an optically read scale impractical for such systems, but the robot may have to tighten a number of fasteners to different amounts of torque making the dial and break type torque wrench also impractical.
Various types of electrical torque measuring systems have been developed. One class of such devices utilizes two alternating current electrical generators positioned at different locations along the driven shaft. An example of such a system is shown in U.S. Pat. No. 2,621,514 entitled "Phase Shift Torque Meter". In this type of device under a no load condition in which the shaft does not have any torque applied to it, the two generated alternating currents are in phase. As torque is developed on the shaft, the two alternating currents shift in phase by an amount that is proportional to the magnitude of torque. A similar system produces two pulsed waveforms which vary in phase with the applied torque.
Another type of torque measuring system employs two L-shaped arms fastened at two locations along the shaft and extending toward each other providing a small gap between the two free ends of the arms. At the end of one arm is a permanent magnet and the end of the other arm holds a Hall effect generator. An example of a device of this type is shown in U.S. Pat. No. 3,191,434 entitled "Device for Measuring Torque on Shafts". Under a no-load condition, the output signal from the Hall generator of this device will be zero. As soon as the magnet is displaced with respect to the Hall generator, as occurs when the shaft is under load, the Hall generator will produce a voltage that is proportional to the torque on the shaft.